1. Field of the Invention
The present invention generally relates to power supplies. More particularly, the present invention relates to the start-up circuit of a switching power supply.
2. Description of the Prior Art
Switching mode power supplies have largely replaced linear transformers and linear power supplies. Switching mode power supplies have grown in popularity, because they are more cost effective than linear power supplies. Furthermore, they offer several other advantages over linear power supplies, including reduced size, improved efficiency, and higher performance.
However, the start-up circuitry commonly used by switching mode power supplies can be substantially improved.
Several passive start-up devices known in the prior arts include U.S. Pat. No. 5,200,886 (Karl Schwarz, Horst Bartussek, Helmut Rettenmaier), U.S. Pat. No. 5,262,933 (Chen Shyi-Hon), U.S. Pat. No. 5,452,195 (Steffen Lehr, Volker Neiss, Jose I. Rodriguez-Duran, Rudolf Koblitz), and U.S. Pat. No. 6,069,805 (Wayne Anderson). The main drawback of these circuits is high power consumption.
Start-up circuits using high-voltage transistors are also well known in the prior arts. Examples of such start-up circuits are disclosed in U.S. Pat. No. 5,200,886 (Karl Schwarz, Horst Bartussek, Helmut Rettenmaier), U.S. Pat. No. 6,002,598 (Erwin G. R. Seinen, Naveed Majid), and U.S. Pat. No. 6,480,402 (Claudio Adragna, Claudio Spini). The drawback of these start-up circuits is also high power consumption because they require primary-side protection circuits.
In recent years, the manufacturers of computers and other types of equipment have been striving to comply with increasingly stringent environmental regulations. US and European regulations regarding electrical appliances strictly limit the amount of power that is consumed by supervising circuits and remote-control circuits. Reducing standby-mode power consumption has become a major concern. The start-up circuits of known power supplies are a major source of power loss. Furthermore, because traditional power supplies typically have high power consumption under light-load and zero-load conditions, it is increasingly difficult to manufacture electrical appliances that are compliant with environmental regulations.
FIG. 1 shows the input circuit of a prior-art switching mode power supply based on U.S. patent application Ser. No. 10/065,530 (Yang Ta-yung). In order to comply with safety regulations, a bleeding resistor 20 is used to discharge the energy that is stored in an EMI filter 10. A bridge rectifier 30 and an input capacitor 40 rectify and filter the AC input source VAC into a DC voltage VIN. A transformer 50 is connected to the input capacitor 40. The transformer 50 is also connected in series with a power transistor 80. A control-circuit 100 is used to regulate the power supply. When the AC input source VAC is applied to the power supply, a start-up capacitor 43 will be charged via a start-up resistor 61. The start-up capacitor 43 provides a supply voltage VCC to power the control-circuit 100. The control-circuit 100 comprises an ON/OFF circuit 105, a line-voltage detector (LVD) 120, a latch circuit 150, a PWM (pulse width modulation) circuit 170, and a protection circuit 190. Once the supply voltage VCC provided by the start-up capacitor 43 exceeds a start-threshold voltage, the ON/OFF circuit 105 will enable the control-circuit 100 to begin pulse width modulation (PWM) operation.
After that, an auxiliary winding of the transformer 50 will power the control-circuit 100 via a diode 65. If the supply voltage VCC drops below a stop-threshold voltage, the ON/OFF circuit 105 will shut down the PWM operation of the control-circuit 100.
The PWM circuit 170 generates a PWM signal to switch the power transistor 80. When the power transistor 80 is switched on, the primary current of the transformer 50 will produce a current-sense voltage VS across a resistor 85. A line current IIN, which can represents the line voltage information, is provided to the line-voltage detector 120. The line-voltage detector 120 accepts the line current IIN via a detection resistor 62 connected to the input capacitor 40.
The control-circuit 100 includes the protection circuit 190. The protection circuit 190 will terminate the PWM signal in response to various protection conditions, including over-voltage protection, over-temperature protection, and over-power protection. The line current IIN and the current-sense voltage VS are used to provide over-power protection. After the protection circuit 190 signals the latch circuit 150, the power supply will be locked in an off state. By disconnecting the AC input source VAC and discharging the input capacitor 40, the latch circuit 150 can be reset, so that it will be ready to restart the power supply. Unfortunately, the input capacitor 40 usually has a large capacitance that may take several minutes to completely discharge. To solve this problem, a resistor 63 and a high-voltage transistor 64 are included to accelerate the discharge of the input capacitor 40. However, the bleeding resistor 20, the start-up resistor 61, and the detection resistor 62 consume significant amounts of power. The power consumption of resistors 61 and 62 is equal to VIN2/R. If an increase in the magnitude of the AC input source occurs, the extra power loss will increase dramatically, especially with a 240V AC input.
Thus, the principle drawback of the power supply shown in FIG. 1 is higher power consumption. Another drawback is the need for extra discharge devices such as the resistor 63 and the high-voltage transistor 64. Adding these parts will further increase the cost of the power supply.